Bolts are a form of threaded fasteners with nuts ,and are used to connect two or more parts together. In civil engineering, they are commonly used for connections in steel and timber structures. The connections normally consist of a combination of bolts and steel plates.

Bolt connections on steel bridges

Advantages of using bolts

• They are low cost.
• Easy to operate.
• They form high strength joints.
• They are readily available.
• Simple to design.

Disadvantages of using bolts

• Proper lubrication is required for operation with time.
• They need to be protect from corrosion.
• More space is required when using bolt connections.

Parts of a bolt

Source: Mikael Häggström, CC0, via Wikimedia Commons

Failure Mechanisms

Bolts transfer both tension and shear forces. Bolts also experience a number of failure mechanisms including; shear, tension and bearing failure. Bolts ,therefore, have to be design to perform safely under all these mechanisms.

Bolt connection under shear

Bolt connection under double shear

Bolt connection under tension

Types of bolts

• Anchor bolts: These kinds of bolts are normally implanted into concrete or masonry to form part of structural connections.
• Lag bolts: Lags are heavy duty fasteners designed to create their own mating threads through wood.
• Machine bolts: These have a short shank and are usually used to hold metal components together with through predrilled holes.
• Carriage bolts: Carriage bolts are completely threaded with a square undercut to the head. They are normally used to fasten timber and metal components.
• U-bolts: U-bolts are bent in form of a “U” shape. They are normally used to support service pipes.
• Hanger bolts: Hanger bolts are made without a head and have lag screw thread at one end with a machine bolt thread on the other. They suitable for fastening timber and metal components together.

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